dcarpentiere, specific system recommendations are difficult to make without the benefit of a site visit to determine the constraints on the site affecting the design, and determining the specific technology which is most appropriate for this application. That said, I will give you some general recommendations for the design of a septic tank/soil absorption system. The design recommendations yield a system that lends itself to “EASY” access and perhaps, (just perhaps mind you) if it is “Really Easy” there is a chance in hell that you will actually perform the required annual maintenance procedures. I will also discuss alternative leach field materials with which I have experience.
Most on-site sewage disposal system regulations in this country prescribe minimum design standards for septic tank/soil absorption systems which yield a system that simply cannot be “Easily” maintained. Most septic system contractors, install the least cost system possible, and many systems are undocumented. The problem with this condition is that the maintenance responsibility is placed with the homeowner, and there is no practical enforcement method, so the systems do not receive maintenance except when the system fails. This is called “Maintenance By Crisis.”
I advocate that the annual maintenance procedures be performed on July 4th, “SEWAGE INDEPENDENCE DAY!” Celebrate your independence of the “Sewer Grid”, but remember that with this independence, comes the responsibility of maintaining a sewage treatment system.
The septic tank should be fitted with 20 inch diameter risers to the ground surface, covering both the inlet end, and outlet end access holes. A plastic Septic Tank Effluent Filter should be installed in the outlet tee of the septic tank.
Sludge depth should be measured annually in the first compartment of the septic tank. When the sludge depth is 1/3 the total liquid depth of the tank, then the sludge should be removed.
The septic tank effluent filter is removed, and the debris which has accumulated on the filter is flushed back into the tank with the strong stream of a garden hose.
Leach Field Design
I am an advocate of alternating leach field design. I design gravelless leach field systems with a diversion valve so that the effluent flow can be diverted to an alternate leach field annually (on SEWAGE INDEPENDENCE DAY). I do not recommend the use of the traditional stone and perforated pipe leach fields. Fifty percent of the required leach field area is in each field.
The thickness of the clogging mat, formed at the surface of the soil where the septic tank effluent is applied is the controlling factor in the rate of percolation of the water going through it. As the clogging mat thickness increases, the percolation rate decreases. At some point in time, (in the case of the lateral line which you described, 4 years)the application rate of the effluent is greater than the percolation rate through the clogging mat, and when this condition occurs the effluent either surfaces or backs up into the septic tank, and possibly backs up into the house.
The long term “resting” of the leach field will allow the soil, and the clogging mat, to become unsaturated, and the environment then changes from an anaerobic condition to an aerobic condition. The clogging mat is then consumed by the aerobic soil microbes, and the iron compounds (Ferrous sulfide) are oxidized by the oxygen in the atmospheric air supplied through the ventilation ports.
Monitoring and ventilation ports are installed on each end of each leach field. The in-use field riser is fitted with a solid threaded plug to prevent sewer odors from emanating into the yard. The resting leach field risers are fitted with 4 inch drain grates so that atmospheric air can enter the gravelless leach field which is typically 2 feet below the ground.
The vent caps, and the solid threaded caps are exchanged when the septic tank effluent is diverted from one field to the other.
I recommend that you also prepare a maintenance log, and record the maintenance that has been performed on this system. This log is a valuable sales tool in the future.
Gravelless Leach Field Material
The leach field material that I specify most in my practice here in Colorado, is Advanced Drainage Systems, Inc. (ADS), SB*2 Gravelless Leach Field Tubing.
I also specify the use of the popular plastic “Chamber” leach field material. ADS manufactures a chamber system called the “Bio-Diffuser”. Another popular brand is called the “Infiltrator”. There are a couple of other brands of chamber systems available. I specify the use of a geotextile fabric layer over the top, and covering the louvers on the sides of any chamber leach field. This geotextile, actually landscaping weed barrier fabric, prevents the migration of silt, and clay particles into the void of the chamber. The fabric also acts as a wick which “wicks” the effluent over the top of the chambers thereby increasing the soil infiltrative area.
There are other gravelless leach field systems available, but I have not personnally installed these other sytems.
I hope this rather long diatribe is helpful in making your septic system design decisions. Oh, by the way, I am unaware of anyone guarenteeing the performance of an on-site sewage treatment system. The reasons for this are the fact that most on-site sewage treatment systems are “Designed To Fail,” and the homeowner has control over what material is going into the septic system, and the constituents of the sewage may cause the system to fail. It is difficult to determine the exact cause of system failure, and a failure may be the result of several contributing factors.
If you insist on a septic system design which lends itself to easy maintenance, and then actually perform the required maintenence, chances are that you will not need a guarentee.
Good Luck, JWA
[Edited by John Aldrich on 25 June 2001]